(1) Field of the Invention
The present invention relates to a method of detecting flaws in a knitted fabric including flaws known as runs and circular holes which define broader spaces between adjacent lines of thread than normal stitching spaces, and to an apparatus for carrying out this method.
(2) Description of the Prior Art
An optical sensor comprising a phototransistor, a television camera or the like is employed recently in a device for detecting flaws in a fabric knitted by a knitting machine. Generally, this device detects a defference in quantity level of light transmission or reflection between a normal fabric background and flaws by means of signals provided by the sensor. An appropriate threshold value is set for the quantity levels of light transmission or reflection of the fabric background and flaws, and the quantity levels exceeding the threshold value are judged to identify with the flaws. However, such a device has the disadvantage of failing to detect the flaws accurately owing to variations in the light levels of the fabric background and flaws caused by external lights or the like. Therefore, in a flaw detecting method actually practiced, an average value is taken of first order lag elements of the signals, a difference between this average value and an actual measurement value is determined, and checking is made as to whether the difference exceeds a threshold value or not.
The flaws in the knitted fabric include what is known as a run which defines a space between vertical lines of thread about 50 percent greater than a space defined by normal stitching. The run sometimes is created by a mechanical trouble due to a bent needle or the like, and in such a case the machine must be stopped immediately. However, the run results in smaller light level variations than a flaw in the form of a circular hole. In the method of detecting flaws by classifying light levels by a fixed threshold value, such small light level variations are not judged to represent a flaw signal but are judged to be noise levels attributable to external light. Furthermore, rises of the signal are not very steep, and this renders the method of taking an average value of the signal ineffectual too in that the average value varies following the actual measurement value and, therefore, the difference remains below the theshold value to be judged not to represent a flaw signal. Thus, the known methods described above are unable to detect runs in the knitted fabric as flaws, which has been an obstacle to the progress for knitting machine automation.